Alpha over node is including unwanted objects in the image?
You need to Connect the matte output of the Crytomatte to the Alpha Over Node for it to work as shown in the image. It will get rid of the Asteroids.
Pls pack the textures if you are sending a blend file as the textures go missing as you can see the image.

EDIT
As mentioned in the Comments If you set it up like in the Image you can use Cryptomatte. Also I cleaned up the nodes a bit